Various MEMS devices are becoming increasingly popular. MEMS transducers, including MEMS capacitive receivers, such as microphones, and capacitive transmitters, such as speakers are increasingly being used in portable electronic devices such as mobile telephones and portable computing devices.
Transducer devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. In the case of MEMS pressure sensors and microphones, for example, the read out is usually accomplished by measuring the capacitance between an electrode on a flexible membrane and an electrode on a rigid backplate which will vary as the distance between the electrodes changes as the membrane flexes in response to sound waves incident on the membrane surface.
FIGS. 1a and 1b show a schematic diagram and a perspective view, respectively, of a known capacitive MEMS microphone device 100. The capacitive microphone device 100 comprises a membrane layer 101 which forms a flexible membrane which is free to move in response to pressure differences generated by sound waves. A first electrode 102 is mechanically coupled to the flexible membrane, and together they form a first capacitive plate of the capacitive microphone device. A second electrode 103 is mechanically coupled to a generally rigid structural layer or back-plate 104, which together form a second capacitive plate of the capacitive microphone device. In the example shown in FIG. 1a the second electrode 103 is embedded within the back-plate structure 104.
The capacitive microphone is formed on a substrate 105, for example a silicon wafer which may have upper and lower oxide layers 106, 107 formed thereon. A cavity 108 in the substrate and in any overlying layers (hereinafter referred to as a substrate cavity) is provided below the membrane, and may be formed using a “back-etch” through the substrate 105. The substrate cavity 108 connects to a first cavity 109 located directly below the membrane. These cavities 108 and 109 may collectively provide an acoustic volume thus allowing movement of the membrane in response to an acoustic stimulus. Interposed between the first and second electrodes 102 and 103 is a second cavity 110.
The first cavity 109 may be formed using a first sacrificial layer during the fabrication process, i.e. using a material to define the first cavity which can subsequently be removed, and depositing the membrane layer 101 over the first sacrificial material. Formation of the first cavity 109 using a sacrificial layer means that the etching of the substrate cavity 108 does not play any part in defining the diameter of the membrane. Instead, the diameter of the membrane is defined by the diameter of the first cavity 109 (which in turn is defined by the diameter of the first sacrificial layer) in combination with the diameter of the second cavity 110 (which in turn may be defined by the diameter of a second sacrificial layer). The diameter of the first cavity 109 formed using the first sacrificial layer can be controlled more accurately than the diameter of a back-etch process performed using a wet-etch or a dry-etch. Etching the substrate cavity 108 will therefore define an opening in the surface of the substrate underlying the membrane 101.
A plurality of holes, hereinafter referred to as bleed holes 111, connect the first cavity 109 and the second cavity 110.
As mentioned the membrane may be formed by depositing at least one membrane layer 101 over a first sacrificial material. In this way the material of the membrane layer(s) may extend into the supporting structure, i.e. the side walls, supporting the membrane. The membrane and back-plate layer may be formed from substantially the same material as one another, for instance both the membrane and back-plate may be formed by depositing silicon nitride layers. The membrane layer may be dimensioned to have the required flexibility whereas the back-plate may be deposited to be a thicker and therefore more rigid structure. Additionally various other material layers could be used in forming the back-plate 104 to control the properties thereof. The use of a silicon nitride material system is advantageous in many ways, although other materials may be used, for instance MEMS transducers using polysilicon membranes are known.
In some applications, the microphone may be arranged in use such that incident sound is received via the back-plate. In such instances a further plurality of holes, hereinafter referred to as acoustic holes 112, are arranged in the back-plate 104 so as to allow free movement of air molecules, such that the sound waves can enter the second cavity 110. The first and second cavities 109 and 110 in association with the substrate cavity 108 allow the membrane 101 to move in response to the sound waves entering via the acoustic holes 112 in the back-plate 104. In such instances the substrate cavity 108 is conventionally termed a “back volume”, and it may be substantially sealed.
In other applications, the microphone may be arranged so that sound may be received via the substrate cavity 108 in use. In such applications the back-plate 104 is typically still provided with a plurality of holes to allow air to freely move between the second cavity and a further volume above the back-plate.
It should also be noted that whilst FIG. 1 shows the back-plate 104 being supported on the opposite side of the membrane to the substrate 105, arrangements are known where the back-plate 104 is formed closest to the substrate with the membrane layer 101 supported above it.
The sacrificial material used to define the first and second cavities is dimensioned so as to provide a desired equilibrium separation between the membrane layer 101 and the substrate 105 and also between the membrane layer 101 and the back-plate 104 so as to provide good sensitivity and dynamic range in use. In normal operation the membrane may deform within the volume defined by the first and second cavities without contacting the back-plate and/or substrate 105.
MEMS transducers therefore rely on the electrical property of capacitance between the pair of electrodes. Displacement of the membrane electrode (102) relative to the backplate electrode (103) in response to a sound wave corresponding to a pressure wave incident on the membrane gives rise to a change in distance between the two electrodes. This change in distance can be detected and measured, by means of electronic circuitry (not shown), as a change in capacitance between the two electrodes.
Thus, the measurable change in capacitance is related to the change in distance between the two electrodes according to the relation:Cα1/d
In known MEMS transducers, one electrode is coupled to the flexible membrane and the other electrode is located on, or is part of, a substantially rigid back-plate. As the flexible membrane deflects in response to a pressure differential across the membrane, the membrane electrode is corresponding displaced relative to the substantially stationary back-plate electrode.
It will be appreciated that such an arrangement may suffer from a number of disadvantages. For example, it will be appreciated that the back-plate structure may be a significant source of acoustic noise which is detectable by the capacitive sensor of the transducer. For example, the acoustic holes 112 present in the back-plate 104 may cause noise due to their acoustic resistance. Additionally, the geometry of the acoustic volume between the membrane and the back-plate may cause noise. Also, the electrostatic forces between the back-plate and membrane may vary with displacement, causing non-linearity in the displacement relative to applied acoustic pressure and hence distortion in the electrically sensed output.